Process for post-combustion of reaction gases produced during the vacuum processing of steel

ABSTRACT

A method for after combustion of reaction gases resulting from the decarburization of liquid steel in reaction vessels under vacuum includes the steps of arranging a blow-in opening within a refractory lining of a reaction vessel and introducing an air stream, comprised of hot air of a temperature between 800° C. to 1400° C., counter to the flow direction of the reaction gases via the blow-in opening into the reaction vessel. A device for performing after combustion of reaction gases resulting from the decarburization of liquid steel in a reaction vessel under vacuum includes a generator, connected to the reaction vessel, for producing hot air. The generator contains a bulk of balls for heating the air guided through the bulk of balls and subsequently introduced into the reaction vessel. The bulk of balls consists of a refractory material and is heated by heat energy.

BACKGROUND OF THE INVENTION

The invention relates to a method for after combustion of reaction gasesresulting from the decarburization of liquid steel in reaction vesselsunder vacuum.

In DE 41 30 590 C2 a degassing vessel is provided as a reaction vesselfor the vacuum treatment of liquid steel. In this printed document it isdisclosed that particles are entrained by a degassing current of thereaction gases which leads to a pronounced deposit formation of steelsplashes in the upper part of the reaction vessel and in the area of theconnecting line to the vacuum pump. Such steel deposits may have aconsiderable weight and finally may almost close off the upper end ofthe reaction vessel so that, in general, such steel deposits must beremoved in a complicated manner by melting.

For avoiding the formation of such steel deposits, a method is suggestedin EP 0 347 884 B1, which defines the closest prior art, to the instantinvention with which an after combustion of the resulting reaction gasesis desired. In the context of this known method, oxygen or an oxygencontaining gas is blown via a pipe insertable to a defined spacing abovethe surface of the liquid steel within the steel bath in an amount to becalculated. With this known method three effects are to be achievedjointly, i.e., the decarburization of the steel by supplying oxygen, aheating of the steel bath, as well as an after combustion of thereaction gases resulting from the vacuum treatment. In practice, it wasshown that with the known method the prevention of steel depositsespecially in elongate, respectively, tall reaction vessels cannot beprevented sufficiently reliably.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to improve a method of theaforementioned kind such that the danger of formation of steel depositsin the reaction vessel can be further reduced. Furthermore, a device isto be disclosed that is suitable for performing the improved method.

The solution to this object, including advantageous embodiments anddevelopments, can be taken from the disclosure of the specipication.

The invention is based on the principle that a stream of air isintroduced counter to the flow direction of the reaction gases via theblow-in opening into the reaction vessel, whereby the blow-in opening isarranged within the refractory lining. With the invention it isadvantageously achieved that due to the supply of air an excellent aftercombustion of the reaction gases takes place so that, due to theresulting heat, the formation of steel deposits can be prevented.

According to a preferred embodiment of the invention, it is suggestedthat the air is introduced as hot air with a temperature between 800° C.to 1400° C. This has the advantage that the hot air, due to its own highblow-velocity, has a substantially higher energy impulse and accordinglypenetrates in the direction of length/height of the reaction vessel,counter to the flow direction of the reaction gases, very deeply intothe reaction vessel. This results in a sufficiently strong mixingturbulence of the reaction gases and the hot air stream which results inan improved combustion of the reaction gases and in an improved heattransfer to the inner side of the wall of the reaction vessel. Thereason for this is that the air under conventional conditions can beintroduced into the reaction vessel only with maximally the speed ofsound. For cold air this speed of sound is only 330 m/sec. while thespeed of sound for air at a temperature of, for example, 1200° C. isapproximately 800 m/sec. By using hot air, it is thus possible tointroduce the air into the reaction vessel at substantially increasedvelocity.

In a simplified manner, the introduction of air into the reaction gasesin the reaction vessel results in an elongate, large flame which can becontrolled by the amount of air as well as by the velocity of air beingblown in. By providing such a large flame it is possible to melt quicklyeven existing large steel deposits. Advantageously, the exhaust gases ofthe after combustion of the reaction gases, resulting from theintroduction of air, are relatively cold so that the exhaust gastreatment of the exhaust gases removed from the reaction vessel is alsosimplified.

According to one embodiment of the invention, the introduced amount ofair is calculated such that the amount of reaction gases, calculatedbased on the amount of the steel batch to be degassed, is combustedcompletely stoichiometrically. It is understood that for achieving thisgoal the amount of air to be blown in must be matched to the amount ofevolving reaction gases.

According to one embodiment of the invention, it is suggested that theintroduction of air is carried out for the entire time period of vacuumdegassing of the liquid steel. This measure ensures that during thevacuum treatment of the steel an exhaust gas can be produced that issubstantially free of CO.

Since it is well known that within a first time period, for example,within the first three minutes of a vacuum decarburization that takesapproximately 12 minutes, approximately 50% of the reaction gases arealready removed and that during the following three minutes another 25%are removed, it is expedient according to one embodiment of theinvention to concentrate the introduction of air onto the first timeperiod of the vacuum treatment of the liquid steel whereby this firsttime period may correspond to the first half of the entire treatmentperiod.

According to alternative embodiments of the invention, it is suggestedthat the introduction of air into the reaction vessel is carried outonly for every second or third batch because it may be desirable thatfor the protection of the refractory lining of the reaction vessel athin steel coat should remain on the vessel wall.

In a manner known per se, the inventive method for after combustion ofthe reaction gases can also be combined with an accelerateddecarburization treatment of the liquid steel in which oxygen isintroduced into that steel bath via an insertable pipe.

An expedient device for performing this method is designed such that asuitable generator for generating the hot air is provided and that thegenerator inventively comprises a bulk of balls heatable by heat energyand comprised of a refractory material for the purpose of heating theair to be guided through the bulk of balls. For heating the bulkconsisting of balls a separate burner may be provided according to oneembodiment of the invention but it is also possible that the generatoris connected to the reaction vessel for waste heat recovery so that thehot air resulting after the treatment can be used for heating the bulkconsisting of balls.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows embodiments of the invention which will be describedin the following. It is shown in:

FIG. 1 a schematic representation of a reaction vessel duringintroduction of hot air;

FIG. 2 a diagram showing the ratio of formed reaction gases and theamount of air blown over the treatment period;

FIG. 3 a generator for hot air generation in connection with thereaction vessel in a schematic representation;

FIG. 4 the generator of FIG. 3 in another embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As can be seen in FIG. 1, the reaction vessel 10 is provided at itslower end with two immersion tubes 11 with which the reaction vessel canbe connected to the steel reservoir in which the liquid steel iscontained. When vacuum is supplied by a vacuum pump to the reactionvessel via connector 16, the steel bath 12 will rise in the direction ofarrow 13 from the non-represented steel reservoir and enter the reactionvessel 10. After corresponding treatment, respectively, degassing, itflows in the direction of arrow 14 back into the steel reservoir. Duringthis treatment, reaction gases 15 leave the steel bath 12 and flow inthe direction of the connector opening 16 or to the vacuum pump.

At the upper lid of the reaction vessel 10 a blow-in opening 17 isprovided via which, in the shown embodiment, hot air is blown into thereaction vessel 10, respectively, via which hot air is sucked into thereaction vessel 10 by the vacuum present therein, whereby a flame 19 isformed away from the blow-in opening 17. It is surrounded by the hot aircolumn 18, respectively, it extends therein. The conditions shown inFIG. 1 are based on the hot air blown in at a velocity of 600 m/sec. andat a flow velocity of the reaction gases of 15 m/rsec., whereby for atotal height of the reaction vessel of 10 to 12 m the hot air canpenetrate deeply into the reaction vessel 10 and thus ensure heattransfer into the lower area of the reaction vessel.

FIG. 2 shows the corresponding vacuum treatment, respectively, hot airintroduction whereby respectively the amount of reaction gas or of hotair is shown as a function of the treatment period. This representationis based on a vacuum treatment of a steel batch of 280 metric tons. Thecurve 20 shows the amount of removed reaction gas for the treatmentperiod of approximately 12 minutes. The hot air is introduced at atemperature of 1200° C. in an amount shown in to the curve 21 along theaxis of time whereby in the represented embodiment the introduction ofhot air is limited to half of the treatment period, i.e., to sixminutes. The measured exhaust gas temperature was approximately 1800° C.and, based on this, energy in the amount of 0.88 GJ is available formelting steel deposits. This is sufficient to melt away a steel depositof approximately 1.5 metric tons.

FIG. 3 shows an expedient generator arrangement for generating hot airwhereby the respective generator 22 is connected by connecting line 23to the blow-in opening 17 for the hot air to be introduced into thereaction vessel 10. The connecting line 23 can be closed off by valve24.

The generator 22 comprises a bulk 25 of balls consisting of refractorymaterial whereby for heating the bulk of balls 25 a separate burner 26is provided which can be operated by gas. The burner 26 is alsoconnected to the connecting line 23. An air supply line 27 extends intothe generator which branches to form an exhaust gas line 28b that can beshut off by a valve 29 and an inlet line 30 that can be shut off by avalve 31.

During heating of the bulk of balls 25, the valve 24 as well as thevalve 31 of the inlet line 30 are closed. Thus, the hot exhaust gasesintroduced by the gas burner 26 can flow through the bulk of balls 25and can exit via the exhaust gas line 28 when the valve 29 is opened.For the introduction of hot air, the valve 29 is closed and the valves31 and 24 are opened. Due the vacuum present within the reaction vessel10, the air can be introduced via the lines 30 and 27 into the generator22 and is heated by the heated bulk of balls 25 to the desiredtemperature. The heated hot air is then introduced via the connectingline 23, when the valve 24 is open, into the reaction vessel 10 via theblow-in opening 17. It is expedient that the connecting line 23 betweenthe generator 22 and the reaction vessel 10 is as short as possible.Furthermore, the blow-in opening 17 within the reaction vessel is to bedimensioned such that for the respectively present inner pressure,respectively, vacuum within the reaction vessel the best possible flowconditions for the introduction of hot air are provided.

In the embodiment represented in FIG. 4, the recovery of waste heatwithin the reaction vessel is suggested whereby the air supply line 27branches into an inlet line 30 and into a connecting line 32 extendingto the reaction vessel 10. The connecting line 32 can be closed off by avalve 33, and a suction fan 34 may also be provided in this line. Inthis embodiment, the valve 24 is no longer arranged within theconnecting line 23 between the generator 22 and the reaction vessel 10,but is positioned within the air supply line 27.

In this embodiment the heating of the bulk of balls 25 by passing in hotair provided within the reaction vessel 10 is carried out when the valve24 and the valve 33 are open and the suction fan 34 is running, whereby,after heating of the filling 25, the valve 33 is closed while the valve31 within the inlet line is opened so that the air can enter via the airsupply line 27 into the heated bulk of balls 25. From here, it can flowvia the connecting line 23 to the reaction vessel 10. In bothembodiments, the amount of hot air to be introduced into the reactionvessel 10 can be controlled as a function of the vacuum present withinthe reaction vessel 10 by the valve 24.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What is claimed is:
 1. A method for after combustion of reaction gasesresulting from the decarburization of liquid steel in reaction vesselsunder vacuum, including the steps of:arranging a blow-in opening withina refractory lining of a reaction vessel; and introducing an air stream,comprised of hot air of a temperature between 800° C. to 1400° C.,counter to the flow direction of the reaction gases via the blow-inopening into the reaction vessel.
 2. A method according to claim 1,wherein in the step of introducing the amount of air of the air streamis calculated such that the amount of reaction gases calculated based onthe steel batch to be degassed is burnt completely stoichiometrically.3. A method according to claim 1, wherein the step of introducing iscarried out over the entire time period of decarburization of the liquidsteel under vacuum.
 4. A method according to claim 1, wherein the stepof introducing is limited to a first portion of the time period ofdecarburization of the liquid steel under vacuum, wherein the firstportion is limited to half the time period of decarburization.
 5. Amethod according to claim 1, wherein the method is used for batches ofliquid steel and wherein the step of introducing is carried out forevery other batch.
 6. A method according to claim 1, wherein the methodis used for batches of liquid steel and wherein the step of introducingis carried out for every third batch.
 7. A method according to claim 1,further comprising the step of blowing oxygen into the liquid steel viaa pipe that can be inserted into the reaction vessel for an accelerateddecarburization of the liquid steel.
 8. A device for performing aftercombustion of reaction gases resulting from the decarburization ofliquid steel in a reaction vessel under vacuum, said device comprising:agenerator for producing hot air connected to the reaction vessel; saidgenerator comprising a bulk of balls for heating the air guided throughsaid bulk of balls and subsequently introduced into the reaction vessel;said bulk of balls comprised of a refractory material and heated by heatenergy.
 9. A device according to claim 8, comprising a separate burnerfor heating said bulk of balls.
 10. A device according to claim 8,wherein said generator is connected to the reaction vessel for wasteheat recovery to be used for heating said bulk of balls.